Preprint: Electron g-factor of valley states in realistic silicon quantum dots

We theoretically model the spin-orbit interaction in silicon quantum dot devices, relevant for quantum computation and spintronics. Our model is based on a modified effective mass approach with spin-valley boundary conditions, derived from the interface symmetry under presence of perpendicular to the interface electric field. The g-factor renormalization in the two lowest valley states is explained by the interface-induced spin-orbit 2D (3D) interaction, favoring intervalley spin-flip tunneling over intravalley processes. We show that the quantum dot level structure makes only negligible higher order effects to the g-factor. We calculate the g-factor as a function of the magnetic field direction, which is sensitive to the interface symmetry. We identify spin-qubit dephasing sweet spots at certain directions of the magnetic field, where the g-factor renormalization is zeroed: these include perpendicular to the interface magnetic field, and also in-plain directions, the latter being defined by the interface-induced spin-orbit constants. The g-factor dependence on electric field opens the possibility for fast all-electric manipulation of an encoded, few electron spin-qubit, without the need of a nanomagnet or a nuclear spin-background. Our approach of an almost fully analytic theory allows for a deeper physical understanding of the importance of spin-orbit coupling to silicon spin qubits.

Electron g-factor of valley states in realistic silicon quantum dots (arxiv.org)



Preprint: Quantum-limited measurement of spin qubits via curvature coupling to a cavity

We investigate coupling an encoded spin qubit to a microwave resonator via qubit energy level curvature versus gate voltage. This approach enables quantum non-demolition readout with strength of tens to hundred MHz all while the qubit stays at its full sweet-spot to charge noise, with zero dipole moment. A “dispersive-like” spin readout approach similar to circuit-QED but avoiding the Purcell effect is proposed. With the addition of gate voltage modulation, selective longitudinal readout and n-qubit entanglement-by-measurement are possible.

Quantum-limited measurement of spin qubits via curvature coupling to a cavity (arxiv.org)



PRB: Entangling distant resonant exchange qubits via circuit quantum electrodynamics

We investigate a hybrid quantum system consisting of spatially separated resonant exchange qubits, defined in three-electron semiconductor triple quantum dots, that are coupled via a superconducting transmission line resonator. Drawing on methods from circuit quantum electrodynamics and Hartmann-Hahn double resonance techniques, we analyze three specific approaches for implementing resonator-mediated two-qubit entangling gates in both dispersive and resonant regimes of interaction. We calculate entangling gate fidelities as well as the rate of relaxation via phonons for resonant exchange qubits in silicon triple dots and show that such an implementation is particularly well suited to achieving the strong coupling regime. Our approach combines the favorable coherence properties of encoded spin qubits in silicon with the rapid and robust long-range entanglement provided by circuit QED systems.

Entangling distant resonant exchange qubits via circuit quantum electrodynamics (aps.org)



Talk: Dogs and cats, living together! How approaches to Josephson junction and spin-based quantum computing can learn from each other

I will talk about two of our recent proposals for quantum computers based on superconducting JJ circuits [1] and spins in semiconductors [2]: what motivated them, why they are worth pursuing experimentally, and directions for future research.

[1] Semiconductor-inspired design principles for superconducting quantum computing, Nature Communications 7, 11059 (2016)

[2] Charge-noise-insensitive gate operations for always-on, exchange-only qubits, Phys. Rev. B 93, 121410(R) (2016)

Lincoln Laboratory, Boston, MA




PRB-Rapid: Charge-noise-insensitive gate operations for always-on, exchange-only qubits

We introduce an always-on, exchange-only (AEON) qubit made up of three localized semiconductor spins that offers a true “sweet spot” to fluctuations of the quantum dot energy levels. Both single- and two-qubit gate operations can be performed using only exchange pulses while maintaining this sweet spot. We show how to interconvert this qubit to other three-spin encoded qubits as a resource for quantum computation and communication.

Charge-noise-insensitive gate operations for always-on, exchange-only qubits (aps.org)



Talk: Silicon Quantum Information Technology

Information technology based on the fundamental nature of the universe, namely quantum physics, can in some cases dramatically outperform the best “classical” solution. In other words, a quantum computer will be important someday. But the challenges are still immense. Somewhat surprisingly, silicon may continue to be an exceptionally relevant material even into a future era of quantum-enhanced technology. Here I will discuss progress in silicon quantum computing and related semiconductor-based devices touching on my own research interests and highlighting experimental results across the community.

GOMACTech, Orlando, FL



Nature Communications: Semiconductor-inspired design principles for superconducting quantum computing

Superconducting circuits offer tremendous design flexibility in the quantum regime culminating most recently in the demonstration of few qubit systems supposedly approaching the threshold for fault-tolerant quantum information processing. Competition in the solid-state comes from semiconductor qubits, where nature has bestowed some very useful properties which can be utilized for spin qubit-based quantum computing. Here we begin to explore how selective design principles deduced from spin-based systems could be used to advance superconducting qubit science. We take an initial step along this path proposing an encoded qubit approach realizable with state-of-the-art tunable Josephson junction qubits. Our results show that this design philosophy holds promise, enables microwave-free control, and offers a pathway to future qubit designs with new capabilities such as with higher fidelity or, perhaps, operation at higher temperature. The approach is also especially suited to qubits based on variable super-semi junctions.

Semiconductor-inspired design principles for superconducting quantum computing (nature.com)

Press Release: A microwave-free approach to superconducting quantum computing uses design principles gleaned from semiconductor spin qubits. (eurekalert.org)



Preprint: Entangling distant resonant exchange qubits via circuit quantum electrodynamics

We investigate a hybrid quantum system consisting of spatially separated resonant exchange qubits, defined in three-electron semiconductor triple quantum dots, that are coupled via a superconducting transmission line resonator. Drawing on methods from circuit quantum electrodynamics and Hartmann-Hahn double resonance techniques, we analyze three specific approaches for implementing resonator-mediated two-qubit entangling gates in both dispersive and resonant regimes of interaction. We calculate entangling gate fidelities as well as the rate of relaxation via phonons for resonant exchange qubits in silicon triple dots and show that such an implementation is particularly well-suited to achieving the strong coupling regime. Our approach combines the favorable coherence properties of encoded spin qubits in silicon with the rapid and robust long-range entanglement provided by circuit QED systems.

Entangling distant resonant exchange qubits via circuit quantum electrodynamics (arxiv.org)



Talk: A new look at encoded-qubit quantum dot quantum computing in silicon

Although the properties of spin-based qubits are specified by the material system they reside in, it’s possible to modify those properties by encoding a qubit into multiple physical spins. Here we consider new operating regimes for encoded spin qubits and discuss their relevance to spin-based quantum computing and qubit-qubit coupling, especially in silicon quantum dot systems. We will also briefly discuss recent developments in g-factor theory in silicon quantum dots and their possible implications.

We introduce an always-on, exchange-only qubit made up of three localized semiconductor spins that offers a true “sweet spot” to fluctuations of the quantum dot energy levels. Both single- and two-qubit gate operations can be performed using only exchange pulses while maintaining this sweet spot. We show how to interconvert this qubit to other three-spin encoded qubits as a new resource for quantum computation and communication.

APS March Meeting, Baltimore, MD

A new look at encoded-qubit quantum dot quantum computing in silicon




Preprint: Charge-noise-insensitive gate operations for always-on, exchange-only qubits

We introduce an always-on, exchange-only qubit made up of three localized semiconductor spins that offers a true “sweet spot” to fluctuations of the quantum dot energy levels. Both single- and two-qubit gate operations can be performed using only exchange pulses while maintaining this sweet spot. We show how to interconvert this qubit to other three-spin encoded qubits as a new resource for quantum computation and communication.

Charge-noise-insensitive gate operations for always-on, exchange-only qubits (arxiv.org)



NEQST RFI released. Responses due Jan 19

The U.S. Army Contracting Command, Aberdeen Proving Ground, Research Triangle Park Division. is issuing this Request for Information (RFI) in support of the U.S. Army Research Office (ARO) and the Laboratory for Physical Sciences (LPS), in seeking information on emerging ideas, concepts, and approaches for qubit-related science and technology, specifically those related to quantum information processing. Of specific interest are qubit systems that explore new operating regimes and environments, fundamentally new methods of fabrication, and new methods of design, control, or operation. These explorations should have in mind the development of quantum hardware where the novel properties of these systems create significant advantages in coherence, fabrication, and/or qubit operation over current state-of-the-art qubits.

New and Emerging Qubit Science and Technology (NEQST) RFI (fbo.gov)



PRB-Rapid: Spin-orbit coupling and operation of multivalley spin qubits

M. Veldhorst, R. Ruskov, C. H. Yang, J. C. C. Hwang, F. E. Hudson, M. E. Flatté, C. Tahan, K. M. Itoh, A. Morello, and A. S. Dzurak

Spin qubits composed of either one or three electrons are realized in a quantum dot formed at a Si/SiO2 interface in isotopically enriched silicon. Using pulsed electron-spin resonance, we perform coherent control of both types of qubits, addressing them via an electric field dependent g factor. We perform randomized benchmarking and find that both qubits can be operated with high fidelity. Surprisingly, we find that the g factors of the one-electron and three-electron qubits have an approximately linear but opposite dependence as a function of the applied dc electric field. We develop a theory to explain this g-factor behavior based on the spin-valley coupling that results from the sharp interface. The outer “shell” electron in the three-electron qubit exists in the higher of the two available conduction-band valley states, in contrast with the one-electron case, where the electron is in the lower valley. We formulate a modified effective mass theory and propose that intervalley spin-flip tunneling dominates over intravalley spin flips in this system, leading to a direct correlation between the spin-orbit coupling parameters and the g factors in the two valleys. In addition to offering all-electrical tuning for single-qubit gates, the g-factor physics revealed here for one-electron and three-electron qubits offers potential opportunities for different qubit control approaches.

Spin-orbit coupling and operation of multivalley spin qubits (aps.org)



2015 APS Fellow

Grateful to be named a fellow of the American Physical Society by the (future) Division of Quantum Information. Congrats to all the fellows this year!

Tahan, Charles [2015], Laboratory for Physical Sciences

Citation: For important contributions to the field of quantum information science, including theoretical work advancing the experimental development of silicon quantum computers and proposing new quantum devices in the solid state.


Tags career


Preprint: Semiconductor-inspired superconducting quantum computing

Yun-Pil Shim, Charles Tahan

Superconducting circuits offer tremendous design flexibility in the quantum regime culminating most recently in the demonstration of few qubit systems supposedly approaching the threshold for fault-tolerant quantum information processing. Competition in the solid-state comes from semiconductor qubits, where nature has bestowed some almost magical and very useful properties which can be utilized for spin qubit based quantum computing. Here we begin to explore how selective design principles deduced from spin-based systems could be used to advance superconducting qubit science. We take an initial step along this path proposing an encoded qubit approach realizable with state-of-the-art tunable Josephson junction qubits. Our results show that this design philosophy holds promise, enables microwave-free control with minimal overhead (zero overhead in 2-qubit gates), and offers a pathway to future qubit designs with new capabilities such as with higher fidelity or, perhaps, operation at higher temperature. The approach is especially suited to qubits based on variable super-semi junctions.

Semiconductor-inspired superconducting quantum computing (arxiv.org)



Preprint: Spin-orbit coupling and operation of multi-valley spin qubits

M. Veldhorst, R. Ruskov, C.H. Yang, J.C.C. Hwang, F.E. Hudson, M.E. Flatté, C. Tahan, K.M. Itoh, A. Morello, A.S. Dzurak

Spin qubits composed of either one or three electrons are realized in a quantum dot formed at a Si/SiO_2-interface in isotopically enriched silicon. Using pulsed electron spin resonance, we perform coherent control of both types of qubits, addressing them via an electric field dependent g-factor. We perform randomized benchmarking and find that both qubits can be operated with high fidelity. Surprisingly, we find that the g-factors of the one-electron and three-electron qubits have an approximately linear but opposite dependence as a function of the applied dc electric field. We develop a theory to explain this g-factor behavior based on the spin-valley coupling that results from the sharp interface. The outer “shell” electron in the three-electron qubit exists in the higher of the two available conduction-band valley states, in contrast with the one-electron case, where the electron is in the lower valley. We formulate a modified effective mass theory and propose that inter-valley spin-flip tunneling dominates over intra-valley spin-flips in this system, leading to a direct correlation between the spin-orbit coupling parameters and the g-factors in the two valleys. In addition to offering all-electrical tuning for single-qubit gates, the g-factor physics revealed here for one-electron and three-electron qubits offers potential opportunities for new qubit control approaches.

Spin-orbit coupling and operation of multi-valley spin qubits (arxiv.org)



Science Perspective: Catching the quantum sound wave

Rusko Ruskov, Charles Tahan

An ultrasound transducer (“the wand”) both creates and detects sound waves that travel through the body to create images of internal organs or precious cargo (see the figure, panel A). This compact device is made possible with piezoelectric crystals that expand or contract in response to an applied voltage and thus interconvert sound waves and electrical signals. Because sound travels relatively slowly, there is time to process the reflected pulses and display an image in real time. These measurements are in the realm of classical physics, but sound could also play a useful role in quantum-based devices. On page 207 of this issue, Gustafsson et al. (1) take a major step toward that goal, demonstrating a system in which a specially engineered artificial atom in the form of a superconducting quantum bit (qubit) couples to propagating surface acoustic waves on a chip. This soundmatter system shows evidence of quantum behavior.

Catching the quantum sound wave (sciencemag.org)



IEEE: Superconducting-Semiconductor Quantum Devices: From Qubits to Particle Detectors

Yun-Pil Shim, Charles Tahan

Recent improvements in materials growth and fabrication techniques may finally allow for superconducting semiconductors to realize their potential. Here, we build on a recent proposal to construct superconducting devices such as wires, Josephson junctions, and qubits inside and out-of single crystal silicon or germanium. Using atomistic fabrication techniques such as STM hydrogen lithography, heavily doped superconducting regions within a single crystal could be constructed. We describe the characteristic parameters of basic superconducting elements-a 1-D wire and a tunneling Josephson junction-and estimate the values for boron-doped silicon. The epitaxial, single-crystal nature of these devices, along with the extreme flexibility in device design down to the single-atom scale, may enable lower noise or new types of devices and physics. We consider applications for such supersilicon devices, showing that the state-of-the-art transmon qubit and the sought-after phase-slip qubit can both be realized. The latter qubit leverages the natural high kinetic inductance of these materials. Building on this, we explore how kinetic inductance-based particle detectors (e.g., photon or phonon) could be realized with potential application in astronomy or nanomechanics. We discuss supersemi devices (such as in silicon, germanium, or diamond) which would not require atomistic fabrication approaches and could be realized today.

Superconducting-Semiconductor Quantum Devices: From Qubits to Particle Detectors (ieee.org)




Charles Tahan
Physicist in Washington, D.C.